Page 268 - Lindens Handbook of Batteries

11.6 pRIMARy BATTERIES

The following reactions provide a more detailed alkaline cathode reaction scenario.
-
MnO + xH O + xe → MnOOH + xOH (0<x<~0.6) (11.3)
2
2
x
3+
With other products, for x>~0.6 via the soluble Mn species of MnOOH, Mn(OH) , Mn O , and
2
4
3
ZnMn O are produced.
4
2
During the early stage of the cell’s discharge, the anode reaction in KOH produces the soluble
zinc ion [Reaction (11.4)] which can be found in the separator and cathode
-
Zn + 4OH → Zn(OH) 4 –2 + 2e (11.4)
At a certain point in the discharge, depending on the composition of the anode and the rate and
depth of discharge, the electrolyte will become saturated with zincate that then causes the reaction
product to change to the insoluble Zn(OH) . Eventually, the anode will become depleted of water
2
and the zinc hydroxide dehydrates to ZnO by the following two reactions:
-
Zn + 2OH → Zn(OH) + 2e (11.5)
2
Zn(OH) → ZnO + H O (11.6)
2
2
These changes in the different zinc discharge products cannot be easily noted in the discharge
curve since the standard reaction potentials for Reactions (11.5) and (11.6) are very similar. However,
under certain conditions, the formation of the oxide can be sufficiently high that it passivates any
undischarged zinc. Such conditions would include high-rate, low-temperature, and poor electrolyte
conductivity. These concerns are typically mitigated by the use of high surface area zincs in order to
minimize any cell impedance increase by the anode.
The overall total one-electron reaction of the alkaline cell during a continuous discharge is as
follows:
2MnO + Zn + 2H O → 2MnOOH + Zn(OH) (11.7)
2
2
2
Since water is a reactant in Reaction (11.7), the amount of water in a cell is quite important,
especially in high-rate discharge applications. Therefore, the total water management in a cell is an
important variable that battery manufacturers must control in order to provide good performance
over a wide range of discharge conditions. Some battery manufacturers have included additives to
the cell, such as TiO and BaSO , in order to better manage this important characteristic. Also, there
4
2
appear to be many different ZnO morphologies that could affect the anode’s performance.
However, at the low- or intermittent-drain rates, the total cell reaction for 1.33 electrons per
mole is
3MnO + 2Zn → Mn O + 2ZnO (11.8)
2 3 4
This reaction clearly indicates that under such conditions there is no water management
concern.
The open-circuit voltage of an undischarged alkaline cell is typically between 1.55 and 1.65 V,
depending on the purity and activity of the cathode components, the ZnO content of the anode, and
the storage temperature of the cell.
Due to the natural corrosion activity of zinc metal in a basic solution, it can reduce water and
form hydrogen gas. Such a reaction does occur in the alkaline cell and reduces the overall cell
capacity (zinc corrosion) if allowed to become significant. Hydrogen gas evolution can occur during
long-term storage of undischarged cells or after partial discharge. The amount of gas formed during
the latter event depends on the discharge rate, delivered capacity, and storage temperature. This gas
buildup in a cell can cause the cell to bulge and eventually leak. In addition, the formed hydrogen gas
can reduce the manganese dioxide, even further reducing the cell’s available capacity.